Remove the duplicated doc8 executa
Remove the duplicated doc8 executa

the doc8 action run twice when run tox -edocs[1] and -epep8[2],
that's unnecessary, the ps to update it.

[1]:https://github.com/openstack/networking-odl/blob/master/tox.ini#L53
[2]:https://github.com/openstack/networking-odl/blob/master/tox.ini#L92

Change-Id: Iaeab402fb5fd3c6273a57fbb87b187ab01339f42
Closes-Bug: #1800574

openstack/networking-odl,openstack/networking-odl,openstack/networking-odl